The present invention relates generally to a method and apparatus for detecting the presence of particles on the surface of an object and more particularly to a method and apparatus for detecting and measuring the number and sizes of contaminant particles on the surface of an object such as a patterned semiconductor wafer using the principle of scattered light.
Although the invention will hereinafter be described specifically in connection with detecting particles on the surface of a patterned semiconductor wafer, it is to be understood that the invention is not limited to a surface on that particular type of object but rather is useful in detecting the presence of particles on the surface of other objects such as virgin semiconductor wafers, filmed semiconductor wafers and coated or uncoated aluminum memory discs.
In the prior art there are a variety of ways of detecting and measuring the number and sizes of particles on the surface of a semiconductor wafer for the purpose of rejecting those wafers which have on their surface one or more particles above certain sizes or for those having on their surface an excessive number of particles.
One of the most prevalent methods employs the human operator using a light field/dark field microscope. Using the eye, the operator actually counts the number of particles and also identifies the size of the particles, such as those between 1 to 20 microns, and then rejects those wafers which have an excessive number of particles or those having particles of or above a certain size. This method is without doubt highly inaccurate and very expensive both in terms of wages for the human operator and in terms of the number of rejects both after the inspection and after production of the chips (when an erroneously passed wafer is found to have an electrical defect, e.g. short circuits, because of the presence of contaminant particles).
In U.S. Pat. No. 4,377,340 to G. P. Green etc., there is disclosed a method and apparatus for detecting and measuring the number and sizes of impurities on the surface of a material, such as a semiconductor wafer, wherein a beam of high intensity collimated light from a xenon arc lamp is directed onto the surface at normal incidence in the absence of any extraneous light, through a collimating mirror and a pin hole device and whereat the particles will scatter the light, and wherein the surface is viewed by a high light sensitive TV camera which is positioned off-axis to pick up scattered light but not specularly reflected light for display on a viewing screen.
In IBM Technical Disclosure Bulletin Volume 2, No. 10, pages 1672-1673, dated March, 1970, there is disclosed a system for detecting repeated geometric defects on a reflecting surface in which a collimated light beam strikes the surface being examined at a infinite angle of incidence is directed through a telescope to a photomultiplier tube.
In IBM Technical Disclosure Bulletin Volume 21, No. 6, pages 2336-2337 dated November, 1978, there is disclosed a system for detecting defects on wafers wherein light from a plurality of ring light sources impinges on the wafer at an oblique angle to the wafer surface and wherein light scattered upward from the surface at right angles thereto is fed by a lens system into a broad band array detector.
In U.S. Pat. No. 2,947,212 to R. C. Woods there is disclosed a method of detecting surface conditions on a strip of sheet metal having line markings in which light from a light source is directed toward the surface of the sheet metal in a direction generally perpendicular to the line markings. Non-specular reflection in a selected direction which is perpendicular to the lines, and which is preferably between the angle of incidence and the angle of specular reflection, is monitored by a photo-electric cell which is able to detect a surface flaw by variation in the intensity of the reflected light. The light in the incident beam may be polarized and the light in the selected non-specular reflected beam filtered to pass only such polarized light.
In U.S. Pat. No. 4,342,515 to Akiba et al there is disclosed an inspection apparatus for detecting unfavorable foreign matters existent on the surface of an object such as a semiconductor wafer. The apparatus includes a collimated beam generator portion which projects a collimated beam toward the object to-be-inspected from a side thereof and a mechanism which senses light reflected from the surface of the object, through a polarizer plate. In accordance with the disclosed technique for using the apparatus, the signal-to-noise ratio between a detection signal generated by a pattern of the foreign matter to-be-detected and a signal generated by a normal pattern of the object surface and sensed as a noise component can be enhanced.
In U.S. Pat. No. 3,782,836 to Fey et al there is disclosed a surface irregularity analyzing system which includes structure for directing light toward a surface in a direction having a certain angular relationship to the surface. If the light stikes irregularities in the surface it is reflected in a direction having an angular relationship to the surface other than equal and opposite the incident direction. The amount of light reflected from irregularities in the surface is determined, either photographically or photoelectrically, to provide an analysis or irregularities in the surface.
It is an object of this invention to provide a new and improved method and apparatus in detecting the presence of contaminant particles on a surface using the principle of scattered light.
It is another object of this invention to provide a method and apparatus as described above in which the surface being examined is the surface of a semiconductor wafer having a printed circuit pattern formed on it.
It is still another object of this invention to provide a novel optical arrangement for illuminating a surface for the purpose of detecting light scattered by unwanted particles which may be present on the surface.
It is yet still another object of this invention to provide a method and apparatus for detecting particles on a surface which involves illuminating the surface at a grazing angle of incidence with a vertically expanded and horizontally scanning beam of laser light.
It is a further object of this invention to provide a method and apparatus as described above in which the scattered light is detected by a video camera.
It is another object of this invention to provide a method and apparatus as described above wherein unwanted reflected light from the surface and/or from a printed circuit pattern of lines which may be present on the surface is reduced to a minimum.
It is still another object of this invention to provide a method and apparatus as described above in which the surface being examined is illuminated, an area at a time.
It is yet still another object of this invention to provide method and apparatus as described above in which the signal to noise ratio between scattered light from particles and diffracted light from the surface itself is maximized.
It is a further object of this invention to provide a system designed especially for use in dark field illumination applications.
The foregoing and other objects as well as many advantages of the invention will appear from the description to follow. In the description, reference is made to the accompanying drawing which forms a part thereof, and in which is shown by way of illustration, a specific embodiment for practicing the invention. This embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.